This invention relates to a hybrid internal combustion-electric drive system for a vehicle, and specifically to such a drive system that is retrofittable to existing internal combustion engine vehicles. The drive system includes a speed reduction system that enables a standard electric motor to propel the vehicle solely by electric power as a “strong hybrid”. The electric portion of the drive system of this invention is intended to be recharged from an AC power source enabling the vehicle to obtain some or all of its energy from the electric grid and constituting a “plug hybrid”.
There are two major electric hybrid vehicle concepts, series and parallel. In a series hybrid, the wheels are driven solely by the electric motor, which is powered by either the electric storage battery or the IC engine-generator or both. The battery is charged by the generator and by regenerative braking. A series hybrid is essentially an electric vehicle with an on-board engine-generator for battery charging.
In the more sophisticated parallel hybrid, the wheels are driven by both the electric motor and the IC engine. Parallel hybrids are in effect a conventional IC automobile with electric assist. The electric system amplifies the power of the IC engine during acceleration and recovers energy in braking. In this sense the electric system acts like a supercharger to give a small IC engine the performance of a large one. A “strong” parallel hybrid is one in which the electric system can provide enough torque and power that the vehicle can be operated on electric power alone.
The Toyota Prius Synergy drive has achieved a breakthrough in customer acceptance of the parallel hybrid concept, and is the first truly different automotive drive system to achieve fully commercial status since the automatic transmission a half-century ago. It features a planetary-geared system of an IC engine, an electric motor and a motor-generator1. The electric motor drives the wheels directly, and is fixed to the ring gear of the planetary system. The IC engine is fixed to the planet pinion carrier. The sun gear is fixed to the motor-generator. The motor-generator sends excess power from the IC engine to the battery. The electric motor can drive the vehicle by itself at low speeds, and it can augment the torque of the IC engine during acceleration and hill climbing. It can also recover energy from regenerative braking and return it to the battery. By varying the power flow between the two electrical machines, the system acts as a continuously variable ratio, electrically-coupled transmission. The Prius is a strong parallel hybrid up to about 35 miles per hour. 1“The Electric Car”, page 158 and FIG. 10.9, 2001
There is a third hybrid concept, promoted by the Electric Power Research Institute (EPRI) and becoming increasingly popular and well known, called the Plug Hybrid Electric Vehicle (PHEV). This is an attempt to comply with the demand for Zero Emission Vehicles (ZEVs) by providing a parallel hybrid with a larger battery so that it can undertake at least short missions without ever starting the IC engine. The battery is recharged at home from the grid, thereby securing the considerable advantage of low cost energy relative to gasoline. The cycling performance of lithium-ion batteries is being extensively investigated for hybrid use, and it is the plug hybrid that can benefit most from the phenomenal performance of lithium-ion technology. The ability to store substantial amounts of energy for significant range with lithium makes the plug hybrid more than a shopping and commuting ZEV. It will give even a 6000-pound vehicle the range to be all-electric except for the occasional long trip, thereby bringing the economic and environmental advantages of electric propulsion to the entire passenger vehicle fleet.
EPRI is conducting a demonstration program to convert two Daimler Chrysler Sprinter vans to plug hybrids with a 20-30 mile range. This should yield real-world data on the feasibility and operation of plug hybrids in the near future.2 The proposed system interposes an electric motor between a hydraulically operated clutch on the flywheel of the IC engine and the torque converter of an automatic transmission to provide for electric only drive with the clutch released or parallel hybrid operation with the clutch engaged. 2EPRI Portfolio 2006, P 018-001 through -004
Reed et al disclosed a similar approach in U.S. Pat. No. 6,332,257, Dec. 25, 2001, assigned to Chrysler Corp. A conventional manual transmission is converted to a parallel hybrid system by adding an electric motor driving the drive shaft of the transmission through a gear train. A hydraulic clutch and electric shifting actuators convert the manual system to an automatic transmission in which the gears are synchronized by controlling the electric motor speed, thereby achieving the advantage of automatic transmission operation without the use of a hydraulic torque converter with its associated losses. Again the system can operate in an electric-only mode with the clutch released or in a parallel hybrid mode with the clutch engaged. However, the patent envisions only a mild hybrid capability with a small 15 kW electric motor. The main objective is conversion of a mechanical transmission design to a more efficient automatic transmission system with minimal design change.
U.S. patent application 20060000650, Jan. 5, 2006 by Charles E., Hughey discloses a similar system in which the electric motor replaces the output shaft extension of the transmission to augment or replace the power of the internal combustion engine. The intention is to provide retrofit kits to allow existing vehicles to be easily converted to hybrids. It is stated that 15 to 20 variations in length and bolt patterns for the electric motor could replace transmission shaft extensions in over 80% of the light truck market.
U.S. patent application 20050205313, Sep. 22, 2005 by C. D. Gilmore et al describes a simpler system in which the electric motor is built into the driveshaft of a conventional automobile. The shaft contains a circumferentially mounted set of permanent magnets; a stator is mounted on the shaft with ball bearings allowing the shaft to rotate while the stator is constrained against rotation by a link to the vehicle chassis. Power can be applied to the motor from a battery through a controller with inputs from the accelerator, brake and other sensors to the vehicle electronic control module. The system is intended to be easily retrofitable to an existing vehicle as a conversion kit to convert it to a mild hybrid.
The intent of the present invention is similar to Hughey's and Gilmore's, namely to provide a retrofit kit, but with the addition of a speed reduction function to separate the motor from the drive shaft and provide the torque necessary to achieve strong hybrid operation on electricity only. This configuration allows the use of standard motors and other components to accomplish the conversion. Previous proposed attempts to provide conversion kits to create hybrid vehicles required specially designed electric motors, which increases initial cost substantially and requires an inventory of different motor types to convert different IC vehicles. The direct incorporation of the electric motor in the drive train in previous proposals either restricts the torque and power supplied electrically to a minimal augmentation of the IC engine or requires an inordinately large, heavy and expensive electric motor to match the torque output of the IC engine. The provision of a flexible speed reduction system between the electric motor and the drive shaft in the present invention allows a single inexpensive type of electric motor to power a wide variety of vehicles on electric power only. The provision of a flexible speed reduction system such as a timing belt allows for residual motion between the existing drive train due to engine vibration and/or the motion of the wheels over rough ground and the electric motor, which is preferably fixed to the chassis of the vehicle.